Halogen Bonding in the Complexes of Brominated Electrophiles with Chloride Anions: from a Weak Supramolecular Interaction to a Covalent Br–Cl Bond

crystals

Article Halogen Bonding in the Complexes of Brominated Electrophiles with Chloride Anions: From a Weak Supramolecular Interaction to a Covalent Br–Cl Bond

Cody Loy 1, Matthias Zeller 2 and Sergiy V. Rosokha 1,*

1 Department of Chemistry, Ball State University, Muncie, IN 47306, USA; [email protected] 2 Department of Chemistry, Purdue University, West Lafayette, IN 47907, USA; [email protected] * Correspondence: [email protected]

Received: 29 October 2020; Accepted: 23 November 2020; Published: 25 November 2020

Abstract: The wide-range variation of the strength of halogen bonds (XB) not only facilitates a variety of applications of this interaction, but it also allows examining the relation (and interconversion) between supramolecular and covalent bonding. Herein, the Br ... Cl halogen bonding in a series of complexes of bromosubstituted electrophiles (R-Br) with chloride anions were examined via X-ray crystallographic and computational methods. Six co-crystals showing such bonding were prepared by evaporation of solutions of R-Br and tetra-n-propylammonium chloride or using Cl− anions released in the nucleophilic reaction of 1,4-diazabicyclo[2.2.2]octane with dichloromethane in the presence of R-Br. The co-crystal comprised networks formed by 3:3 or 2:2 halogen bonding between R-Br and Cl−, with the XB lengths varying from 3.0 Å to 3.25 Å. Analysis of the crystallographic database revealed examples of associations with substantially longer and shorter Br ... Cl separations. DFT ... computations of an extended series of R–Br Cl− complexes conﬁrmed that the judicious choice of brominated electrophile allows varying halogen Br ... Cl bond strength and length gradually from the values common for the weak intermolecular complexes to that approaching a fully developed covalent bond. This continuity of halogen bond strength in the experimental (solid-state) and calculated associations indicates a fundamental link between the covalent and supramolecular bonding.

Keywords: halogen bonding; bromosubstituted electrophiles; chloride; X-ray crystallography; DFT computations; chemical bonding

1. Introduction Halogen bonding, R-X D, is a directional attraction of electron-rich species D to electrophilic ··· halogen atoms, X (covalently bonded to a group or atom R) [1,2]. Many experimental and computational studies demonstrated that its strength can be varied considerably by changing the nature of XB donors and acceptors [1–7]. The directionality of halogen bonding and the possibility of modulation of its strength open the way to a variety of applications of this intermolecular interaction in areas ranging from crystal engineering of solid-state materials and molecular recognition, to catalysis and rational drug design [1,2,8]. Besides practical applications, the XB variability provides an opportunity to explore the nature of supramolecular bonding and the boundary between such bonding and the covalent bond [6]. Most frequently, halogen bonding (XB) is explained using an electrostatic model which relates this interaction primarily to the attraction of electron-rich nucleophiles to the areas of positive potentials (σ-hole) on the surfaces of the covalently bonded halogen atoms [9]. These areas are formed along the extension of the covalent R–X bonds, and the positive potentials are enhanced by the presence of electron withdrawing substituents in R, as well as by high polarizability of the large bromine and iodine

Crystals 2020, 10, 1075; doi:10.3390/cryst10121075 www.mdpi.com/journal/crystals Crystals 2020, 10, 1075 2 of 14 atoms. This model allows explaining directionality and variation of strength of many XB complexes, especially when the polarization of the reactants is taken into account [10]. Many studies pointed out that besides electrostatic attraction and dispersion, weakly-covalent (charge-transfer) interactions also contribute substantially to the formation of XB complexes [11–17]. These suggestions were supported by the computational analyses of the components of interaction energies [11–13], and by experimental evidence such as unexpected trends in the XB strengths [14], UV–Vis [15] and photoelectron spectra [16,17], structural features [18], and charge-density analysis [19] of the complexes, as well as by a drastic increase in the electron-transfer rates induced by halogen bonding [20–22]. These features are especially pronounced in strongly-bonded complexes. In fact, the XB lengths in some of the complexes are 30–35% shorter than the van der Waals separations of the corresponding atoms [6,23]. Accordingly, they are within 10–15% of the corresponding covalent bonds. In other words, the bond lengths of strong halogen bonds are closer to that of a conventional covalent bond than to weak intermolecular interactions. Such observations raise a fundamental question about the existence and location of a boundary between supramolecular and covalent bonds involving halogen atoms. We have recently demonstrated a gradual change in Br N bond lengths and strengths in a series ··· of XB complexes of brominated electrophiles R-Br with 1,4-diazabicyclo[2.2.2]octane from the values typical for weak intermolecular associates to that characteristic of a covalent bond [6]. In the current work, we present the results of the X-ray structural and computational characterization of XB complexes formed between R-Br electrophiles and Cl− anions. The latter are strong nucleophiles which form complexes with various halogenated electrophiles (and several X-ray structures of such associations are available in the literature, vide infra). Importantly, they allow extending the analysis of bonding in complexes of neutral species to the associations between neutral or cationic XB donor and anionic XB acceptor.

2. Materials and Methods

2.1. Materials Commercially available tetrabromomethane, tribromoﬂuoromethane, hexabromoacetone, 1,4-diazabicyclo[2.2.2]octane (DABCO) and tetra-n-propylammonium chloride ((Pr4N)Cl) (all from Sigma-Aldrich) were used without additional puriﬁcation. Tribromoacetamide was prepared by reaction of hexabromoacetone with NH4OH, and tribromoacetonitrile was synthesized by dehydration of CBr3CONH2 with P2O5 [24]. Tribromonitromethane was synthesized by bromination of nitromethane as described earlier [25].

2.2. Crystallization and X-ray Structural Analysis Single crystals of XB associates suitable for X-ray measurements were obtained by evaporating solutions containing mixtures of an R-Br molecule and (Pr4N)Cl in acetone or by diﬀusion of hexane into dichloromethane solutions containing mixtures of R-Br and DABCO. The latter utilized the Menshutkin reaction between DABCO and CH2Cl2 resulting in formation of Cl− anions which + crystallize with R-Br electrophiles and DABCO-CH2Cl cations (similar reaction was reported earlier by McMurtie and co-workers [26]). For example, 22 mg (0.10 mmol) of (Pr4N)Cl was dissolved together with 33 mg (0.10 mmol) of CBr4 in acetone. Slow evaporation of the solution produced colorless (Pr N)Cl CBr crystals. Crystals of (Pr N)Cl CBr COCBr were obtained in a similar way from an 4 · 4 4 · 3 3 acetone solution containing 54 mg (0.010 mmol) of hexabromoacetone and 22 mg (0.10 mmol) of (Pr N)Cl. Co-crystals of (Pr N)Cl 2CBr CN and (Pr N)Cl CBr F were obtained by evaporation of 4 4 · 3 4 · 3 acetone solutions containing (Pr4N)Cl and an excess of the liquid electrophiles. (Note: X-ray analysis of (Pr N)Cl CBr COCBr and (Pr N)Cl 2CBr CN co-crystals revealed that some Cl anions are replaced 4 · 3 3 4 · 3 − by Br− anions, which were produced by the substitution reaction of Cl− with the R-Br electrophiles.) Co-crystals of (DABCO-CH Cl)Cl CBr NO were obtained by dissolution of 30 mg (0.010 mmol) of 2 · 3 2 Crystals 2020, 10, 1075 3 of 14

CBr3NO2 and 12 mg (0.011 mmol) of DABCO in 5 mL of dry dichloromethane in a Schlenk tube. The resulting solution was carefully layered with 2 mL of a 1:1 dichloromethane/hexane mixture, and then with hexane (20 mL), and kept at 35 C. Slow diffusion of hexane into the dichloromethane − ◦ resulted in the formation of colorless crystals suitable for single crystal X-ray measurements. Co-crystals of (DABCO-CH Cl)Cl CBr CONH were obtained in a similar way using 30 mg (0.010 mmol) of 2 · 3 2 tribromoacetamide instead of tribromonitromethane. The single crystal structures (except (DABCO-CH Cl)Cl CBr NO ) were examined on a Bruker 2 · 3 2 Quest diffractometer with a fixed chi angle, a sealed tube fine–focus X-ray tube, single crystal curved graphite incident beam monochromator, a Photon100 area detector and an Oxford Cryosystems low-temperature device (all X-ray instrumentation was from Bruker AXS Inc., Madison, WI, USA). Data were collected at 150 K. Examination and data collection were performed with Mo Kα radiation (λ = 0.71073 Å). A single crystal of (DABCO-CH Cl)Cl CBr NO was analyzed with a Bruker Kappa 2 · 3 2 APEX CCD area detector diffractometer with (microsource) Cu Kα radiation (λ = 1.54178 Å) at 100 K. Reflections were indexed and processed, and the files were scaled and corrected for absorption using APEX3 [27]. The space groups were assigned using XPREP within the SHELXTL suite of programs [28], the structures were solved by direct methods and refined by full matrix least squares against F2 with all reflections using Shelxl2018 [29,30] using the graphical interface Shelxle [31]. If not specified otherwise, H atoms attached to carbon and nitrogen atoms were positioned geometrically and constrained to ride on their parent atoms, with C-H bond distances of 1.00, 0.99 and 0.98 Å for aliphatic C-H, CH2 and CH3 moieties and N-H distances of 0.88 Å, respectively. Methyl H atoms were allowed to rotate but not to tip to best fit the experimental electron density. Uiso(H) values were set to a multiple of Ueq(C) with 1.5 for CH3, and 1.2 for C-H and N-H units, respectively. X-ray structural data were presented using the Mercury 2020.2.0 program [32]. (Pr4N)Cl CBr3F (1). Chemical formula C12H28N CBr3F Cl, M = 492.54 g/mol. Monoclinic, space · · · 3 group Pn (no. 7), a = 8.3500 (5) Å, b = 8.8217 (5) Å, c = 13.0282 (7) Å, β = 97.1844 (19)◦, V = 952.14 (9) Å , 1 3 Z = 2, T = 150 K, µ(MoKα) = 6.49 mm− ,Dcalc = 1.718 g/cm , 22227 reflections measured, 6401 unique (Rint = 0.047). The final R1 was 0.030 (I > 2σ(I)) and wR2 was 0.065 (all data). (Pr N)Cl CBr (2). Chemical formula C H N CBr Cl, M = 553.45 g/mol. Monoclinic, space 4 · 4 12 28 · 4· group P21/n (no. 14), a = 8.3237 (4), Å, b = 18.7831 (9) Å, c = 12.8399 (6) Å, β = 100.5800 (16)◦, 3 1 3 V = 1973.33 (16) Å ,Z = 4, T = 150 K, µ(MoKα) = 8.28 mm− ,Dcalc = 1.863 g/cm , 42420 reflections measured, 7519 unique (Rint = 0.061). The final R1 was 0.034 (I > 2σ(I)) and wR2 was 0.083 (all data). (Pr N)Cl CBr COCBr (3). Chemical formula C H N C Br O 0.101(Br) 0.899(Cl), M = 4 · 3 3 12 28 · 3 6 · · 757.73 g/mol. Orthorhombic, space group Pbca (no. 61), a = 15.6671 (10) Å, b = 15.0131 (8) Å, 3 1 3 c = 19.9888 (13) Å, V = 4701.6 (5) Å ,Z = 8, T = 150 K, µ(MoKα) = 10.52 mm− , Dcalc = 2.141 g/cm , 89148 reflections measured, 8925 unique (Rint = 0.068). The final R1 was 0.037 (I > 2σ(I)) and wR2 was 0.090 (all data). (Pr N)Cl 2CBr CN (4). Chemical formula C H N 2(C Br N) 0.245(Br) 0.755(Cl), M = 788.22 g/mol. 4 · 3 12 28 · 2 3 · · Monoclinic, space group C2/c (no. 15), a = 8.2066 (6) Å, b = 14.0978 (11) Å, c = 22.966 (3) Å, β = 94.942 (4)◦, 3 1 3 V = 2647.2 (4) Å ,Z = 4, T = 150 K, µ(MoKα) = 9.55 mm− ,Dcalc = 1.978 g/cm , 24848 reflections measured, 4896 unique (Rint = 0.073). The final R1 was 0.050 (I > 2σ(I)) and wR2 was 0.163 (all data). (DABCO CH2Cl)Cl CBr NO (5). Chemical formula CBr NO C H ClN Cl, M = 493.86 g/mol. · · 3 2 3 2· 8 15 · Monoclinic, space group P21/c (no. 14), a = 8.2066 (6) Å, b = 12.2997 (5) Å, c = 15.0315 (6) Å, 3 1 3 β = 90.9993 (14)◦,V = 1517.42 (10) Å ,Z = 4, T = 100 K, µ(CuKα) = 13.09 mm− , Dcalc = 2.162 g/cm , 2643 reflections measured, 2643 unique. The final R1 was 0.048 (I > 2σ(I)) and wR2 was 0.134 (all data). (DABCO-CH2Cl)Cl CBr CONH (6). Chemical formula C H ClN C H Br NO Cl, M = · 3 2 7 14 2· 2 2 3 · 492.88 g/mol. Monoclinic, space group P21/c (no. 14), a = 6.8643 (3) Å, b = 16.6939 (8) Å, c 3 1 = 13.1824 (7) Å, β = 90.029 (2)◦,V = 1510.60 (13) Å ,Z = 4, T = 100 K, µ(MoKα) = 8.36 mm− , 3 Dcalc = 2.167 g/cm , 36132 reflections measured, 5739 unique (Rint = 0.088). The final R1 was 0.050 (I > 2σ(I)) and wR2 was 0.106 (all data). Crystals 2020, 10, 1075 4 of 14

Complete crystallographic data, in CIF format, have been deposited with the Cambridge Crystallographic Data Centre. CCDC 2020744–2020749 contain the supplementary crystallographic data for this paper. These data can be obtained free of charge via www.ccdc.cam.ac.uk/data_request/cif.

2.3. Computations Quantum-mechanical calculations were carried out using the Gaussian 09 suite of programs [33]. Geometries of electrophiles and halogen-bonded R–Br ... Cl complexes were optimized without constraints in the gas phase and in dichloromethane via DFT calculations with the M06-2X functional and the 6-311+G(d,p) basis set [34]. Previous theoretical analyses indicated that such commutations provide excellentCrystals 2020 geometries, 10, x and energies at a reasonable computational cost [35–374 of]. 15 Importantly, our earlier studies demonstrated that characteristics of the XB complexes between R-Br electrophiles 2.3. Computations with halide or pseudohalide anions obtained via such computations were in good agreement with Quantum-mechanical calculations were carried out using the Gaussian 09 suite of programs [33]. experimentalGeometries data [15 of,21 electrophiles,38,39]. Since and halogen-bonded previous works R–Br… showedCl complexes that were the optimized solid-state without environment is best modelledconstraints using in calculations the gas phase and with in dichloromethane a moderately via polar DFT solventscalculations as with the the medium M06-2X functional [15], the geometry optimizationsand in the the 6-311+G(d,p) current work basis set were [34]. done Previous via theoretical computations analyses in indicated the gas that phase such andcommutations in dichloromethane provide excellent geometries and energies at a reasonable computational cost [35–37]. Importantly, (the latter wereour earlier carried studies out demonstrated using a Polarizable that characteristics Continuum of the XB complexes Model between [40]). TheR-Br electrophiles absence of imaginary vibrational frequencieswith halide or pseudohalide conﬁrmed anions that obtained the optimized via such computations structures were represent in good agreement true minima. with Values of experimental data [15,21,38,39]. Since previous works showed that the solid-state environment is best halogen-bond energies ∆E were determined as: ∆E = Ecomp (ER-Br + ECl) + BSSE, where Ecomp,ER-Br modelled using calculations with a moderately polar solvents− as the medium [15], the geometry and E are sums of the electronic and ZPE of the optimized complex, R-Br and chloride, respectively, Cl optimizations in the current work were done via computations in the gas phase and in and BSSE isdichloromethane a basis set superposition (the latter were carried error out [41 usin]. Sinceg a Polarizable formation Continuum of complexes Model [40]). The involve absence deformation of R-Br, theseof imaginary values can vibrational be expressed frequencies as confirmed∆E = ∆ thatEstrain the optimized+ ∆Eint structures, where represent∆Eint is true an minima. interaction energy Values of halogen-bond energies ΔE were determined as: ΔE = Ecomp − (ER-Br + ECl) + BSSE, where Ecomp, between deformed reactants [42]. The ∆Eint values follow the same trends as those of ∆E (see Figure S3 ER-Br and ECl are sums of the electronic and ZPE of the optimized complex, R-Br and chloride, in the Supportingrespectively, Information). and BSSE is Energiesa basis set superpositio and atomicn error coordinates [41]. Since formation of the calculated of complexes complexes involve are listed Δ Δ Δ Δ in the Supportingdeformation Information. of R-Br, these values can be expressed as E = Estrain + Eint, where Eint is an interaction energy between deformed reactants [42]. The ΔEint values follow the same trends as those of ΔE (see 3. Results andFigure Discussion S3 in the Supporting Information). Energies and atomic coordinates of the calculated complexes are listed in the Supporting Information.

3.1. Crystallization3. Results and and X-ray Discussion Structural Characterization of the Solid-State Associates of R-Br with Cl−

Evaporation3.1. Crystallization of a solution and X-ray containing Structural Characteri a mixturezation of of tribromoﬂuoromethane the Solid-State Associates of R-Br andwith Cl Pr− 4NCl (1:1 molar ratio) in acetone resulted in formation of colorless block-like crystals containing both the organic Evaporation of a solution containing a mixture of tribromofluoromethane and Pr4NCl (1:1 molar molecule andratio) the inchloride acetone resulted salt. X-rayin formation structural of colorle analysisss block-like of thesecrystalsmonoclinic containing both co-crystals the organic 1 (Pn space group) revealedmolecule the and presence the chloride of quasi-2Dsalt. X-ray structural networks analysis formed of these by monoclinic brominated co-crystals electrophiles 1 (Pn space and chloride group) revealed the presence of quasi-2D networks formed by brominated electrophiles and chloride anions. Each CBr F molecule in these networks is coordinated (through its three bromine substituents) anions.3 Each CBr3F molecule in these networks is coordinated (through its three bromine − with three Clsubstituents)− anions. with In turn, three Cl each anions. chloride In turn, is each bonded chloride to isthree bonded CBr to three3F molecules. CBr3F molecules. Such Such 3:3 CBr3F/Cl− − 3:3 CBr3F/Cl bonding led to formation of zigzag networks consisting of twelve-membered rings. ... bonding led to formation of zigzag networks consisting of twelve-membered rings. Since C-Br Cl− Since C-Br…Cl− fragments are close to linear, these rings resemble cyclohexane in a chair conformation fragments are(Figure close 1a). to linear, these rings resemble cyclohexane in a chair conformation (Figure1a).

(a) (b)

− Figure 1. (aFigure) 2D 1. network (a) 2D network formed formed by by tribromoﬂuoromethane tribromofluoromethane and Cl and anions Cl in (Pranions4N)Cl·CBr in3F (Pr co- N)Cl CBr F − 4 · 3 co-crystals 1.crystals (Note: 1. (Note: blue blue lines lines show showhalogen halogen bonds.) bonds.) (b) Crystal (b) Crystal lattice in lattice the co-crystals. in the co-crystals.

Crystals 2020, 10, 1075 5 of 14 Crystals 2020, 10, x 5 of 15

− The CBr3F molecules and Cl anions propagated along a axes. The Pr4N++ counter-ions are located The CBr3F molecules and Cl− anions propagated along a axes. The Pr4N counter-ions are located inin thethe channelschannels ofof thethe XBXB networknetwork (Figure(Figure1 b).1b). The intermolecular Br...…Cl distances in the networks formed by CBr3F and chloride anions were The intermolecular Br Cl distances in the networks formed by CBr3F and chloride anions were 3.0820 (10) Å, 3.1266 (9) Å and 3.1728 (10) Å. Such separations are 10–15% smaller than the sum of the 3.0820 (10) Å, 3.1266 (9) Å and 3.1728 (10) Å. Such separations are 10–15% smaller than the− sum of the van der Waals radii of bromine and chlorine of 3.66 Å [43]. The corresponding C-Br...…Cl angles were van der Waals radii of bromine and chlorine of 3.66 Å [43]. The corresponding C-Br Cl− angles were 171.80 (11)o, 168.05(11)/o and 178.51 (11)o. 171.80 (11)◦, 168.05(11)/◦ and 178.51 (11)◦. Analogous co-crystallizations of Pr4NCl with tetrabromomethane produced a salt co-crystal 2 Analogous co-crystallizations of Pr4NCl with tetrabromomethane produced a salt co-crystal 2 which comprised hybrid networks formed by a 3:3 halogen-bonded association between CBr4 and which− comprised hybrid networks formed by a 3:3 halogen-bonded association between CBr4 and Cl anions. They consisted of twelve-membered rings similar to that in the co-crystals with CBr3F Cl− anions. They consisted of twelve-membered rings similar to that in the co-crystals with CBr3F (Figure 2). (Figure2).

Figure 2. Halogen-bonded (XB) network formed by tetrabromomethane and Cl− anions in − (PrFigureN)Cl 2. CBrHalogen-bondedco-crystals 2. (XB) network formed by tetrabromomethane and Cl anions in 4 · 4 (Pr4N)Cl·CBr4 co-crystals 2. The three crystallographically non-equivalent Br ... Cl halogen bond lengths of 3.0650 (7) Å, 3.1769 (7) ÅThe and three 3.2431 crystallographically (7) Å in these co-crystals non-equivalent were in the Br… sameCl halogen range asbond those lengths in the of crystals 3.0650 (7) with Å, CBr3F.3.1769 The(7) Å fourth and 3.2431 bromine (7) atomÅ in these in CBr co-crystals4 formed awere symmetric in the same contact range referred as those to asin Typethe crystals I halogen–halogen with CBr3F. contactThe fourth [1] bromine with a symmetry atom in CBr equivalent4 formed a CBr symmetric4 bromine contact atom referr fromed another to as Type layer I halogen–halogen (Figure2). The + BrcontactBr distance [1] with isa symmetry 3.3711 (7) Åequivalent and both CBr C-Br4 bromineBr angles atom are from 151.62 anothe (7) .r Thelayer Pr (FigureN counter-ions 2). The Br⋅⋅⋅ inBr ··· ··· ◦ 4 thesedistance crystals is 3.3711 ﬁlled (7) the Å channels and both formed C-Br⋅⋅⋅Br by angles stacking are of 151.62 the XB (7) layerso. The propagating Pr4N+ counter-ions along the inb theseaxis. Incrystals should filled be mentioned the channels in thisformed respect by stacking that previous of the worksXB layers showed propagating the existence along of the two b axis. types In of should close halogen–halogenbe mentioned in this contacts respect with that a clearprevious geometric works and showed chemical the existence distinctions. of two All types R-Br ofCl close− interactions halogen– ··· − describedhalogen contacts herein follow with descriptiona clear geometric of Type IIand contacts, chemical which distinctions. occur between All R-Br a nucleophile⋅⋅⋅Cl interactions and the positively-chargeddescribed herein follow areas description on the surface of Type of XB II contacts, donor. These which XB occur contacts between are a characterized nucleophile and by thethe R-Brpositively-chargedC− angles of approximatelyareas on the surface 180◦. In of comparison, XB donor. These Type IXB contacts contacts represent are characterized interactions by between the R- ···− identicalBr⋅⋅⋅C angles areas of on approximately the surfaces of 180 twoo. In covalently-bonded comparison, Type halogens I contacts in represent the two R-Xinteractions molecules, between with theidentical equal areas R-X onX angles the surfaces (usually of two in the covalently-b 150 –160 ondedrange). halogens Resnati, in Metrangolo the two R-X et molecules, al. pointed with out the [1] ··· ◦ ◦ thatequal (symmetrical) R-X⋅⋅⋅X angles Type (usually I contacts in the are 150 relatedo–160 too range). close-packing Resnati, requirements Metrangolo et and al. they pointed are notout halogen [1] that bonding(symmetrical) (which Type occur I betweencontacts electrophilicare related to and close-packing nucleophilic requirements sites according and to thethey IUPAC are not deﬁnition). halogen bondingCrystallization (which occur from between a solution electrophilic containing a mixtureand nucleophilic of Pr4NCl andsites hexabromoacetoneaccording to the resultedIUPAC indefinition). formation of co-crystals 3 showing quazi-2D layers formed by 3:3 halogen bonding between the brominatedCrystallization electrophile from anda solution the anion containing (Figure 3a). mixture In contrast of Pr4 toNCl the and layers hexabromoa describedcetone above, resulted they consistedin formation of two of typesco-crystals of cells. 3 showing The smaller quazi-2D cells were layers formed formed by pairsby 3:3 of halogen CBr3COCBr bonding3 and between pairs of Clthe−. Thebrominated larger cells electrophile comprised and four the XB anion donors (Figure linked 3). by In four contrast chloride toanions the layers (Figure described3). above, they consisted of two types of cells. The smaller cells were formed by pairs of CBr3COCBr3 and pairs of Cl−. The larger cells comprised four XB donors linked by four chloride anions (Figure 3).

Crystals 2020, 10, x 6 of 15 CrystalsCrystals 20202020, 10, ,10 x , 1075 6 of6 15 of 14

− FigureFigure 3. 3. XBXB network network formed formed by by hexabromoacetone hexabromoacetone and and Cl− Cl anions− anions in (Pr in4 (PrN)Cl·CBr4N)Cl CBr3COCBr3COCBr3 co-3 Figure 3. XB network formed by hexabromoacetone and Cl anions in (Pr4N)Cl·CBr3COCBr· 3 co- co-crystals. crystals.crystals. ... The XB Br… Cl lengths measured in these crystals are 3.0306 (8) Å, 3.0618 (8) Å and 3.2056 (8) Å. The XB Br… Cl lengths measured in these crystals are 3.0306 (8) Å, 3.0618 (8) Å and 3.2056 (8) Å. The XB Br... Cl lengths measured in these crystals are 3.0306... (8) Å, 3.0618 (8) Å and 3.2056 (8) Å. Besides Br… Cl bonds, these crystals also show short Br… Br contacts of 3.5683 (8) Å between BesidesBesides Br Br…ClCl bonds, bonds, these these crystals crystals also also show show short short Br Br…BrBr contacts contacts of of 3.5683 3.5683 (8) (8) Å Åbetween between neighboringCBr CBr3COCBr3COCBr3 3 molecules. neighboringneighboring CBr 3COCBr3 molecules. molecules. Evaporation of a solution containing an excess of tribromoacetonitrile and Pr4 NCl produced EvaporationEvaporation of of a asolution solution containing containing an an excess excess of of tribromoacetonitrile tribromoacetonitrile and and Pr Pr4NClNCl produced produced monoclinic crystals 4 (P211/c space group). They contained XB donors and acceptors in a 2:1 ratio monoclinicmonoclinic crystals crystals 4 4(P2 (P21/c /cspace space group). group). They They contained contained XB XB donors donors and and acceptors acceptors in in a a2:1 2:1 ratio ratio − formingforming layers layers parallel parallel to to the ab plane. These These layers layers consist consist of columns of Cl− − anionsanions and and double forming layers parallel to the ab plane. These layers consist of columns+ of Cl anions and double columns of CBr CN molecules, and they are separated by the Pr N+ counter-ions. Within the layers, columns of CBr33CN molecules, and they are separated by the Pr4N+ counter-ions. Within the layers, columns of CBr3CN molecules, and they are separated by the Pr4N counter-ions.... Within the layers, each tribromoacetonitrile is bonded with three Cl − anions, with Br … Cl bond lengths of 3.0980 (4) Å, eacheach tribromoacetonitrile tribromoacetonitrile is isbonded bonded with with three three Cl Cl− −anions, anions, with with Br Br…ClCl bond bond lengths lengths of of 3.0980 3.0980 (4) (4) Å, Å, 3.1320 (4) Å and 3.1398 (4) Å (Figure4a). In turn, each chloride coordinates six bromine atoms 3.13203.1320 (4) (4) Å Åand and 3.1398 3.1398 (4) (4) Å Å(Figure (Figure 4a). 4a). In In turn, turn, ea eachch chloride chloride coordinates coordinates six six bromine bromine atoms atoms (Figure (Figure (Figure4b). 4b).4b).

(a()a ) (b()b )

Figure 4. (a) XB networks formed by tribromoacetonitrile and chloride anions in (Pr4N)Cl CBr3CN Figure 4. (a) XB networks formed by tribromoacetonitrile and chloride anions in (Pr4N)Cl·CBr· 3CN co- Figure 4. (a) XB networks formed by tribromoacetonitrile and chloride anions in (Pr4N)Cl·CBr3CN co- co-crystals 4. (b) Fragment of the X-ray structure of co-crystals 4 showing a (hexacoordinated)− Cl− crystalscrystals 4. 4.(b ()b Fragment) Fragment of of the the X-ray X-ray structure structure of of co-crystals co-crystals 4 showing4 showing a (hexacoordinated)a (hexacoordinated) Cl Cl− anion anion anion bonded with six CBr3CN molecules. bonded with six CBr3CN molecules. bonded with six CBr3CN molecules. Diﬀusion of hexane into a dichloromethane solution containing 1,4-diazabicyclo[2.2.2]octane (DABCO)DiffusionDiffusion and of eitherof hexane hexane tribromoacetamide into into a adichloromethane dichloromethane or tribromonitromethane solu solutiontion containing containing resulted 1,4-diazabicyclo[2.2.2]octane 1,4-diazabicyclo[2.2.2]octane in formation of co-crystals 5 (DABCO)(DABCO) and and either either tribromoacetami tribromoacetamidede or or tribromonitromethane tribromonitromethane+ resulted resulted in in formation formation of of co-crystals co-crystals and 6 containing R-Br, Cl−−anions and the DABCO-CH2Cl counter-ions. Such crystallization utilized − 2 + 5 and5 and 6 containing6 containing R-Br, R-Br, Cl Cl anions anions and and the the DABCO-CH DABCO-CH2ClCl+ counter-ions.counter-ions. Such Such crystallization crystallization utilized utilized the Menshutkin reaction between DABCO and CH2Cl2 (producing Cl−−) in the presence of the XB the Menshutkin reaction between DABCO and CH2Cl2 (producing Cl− ) in+ the presence of the XB thedonors Menshutkin [26]. Co-crystals reaction between 5 of CBr3 NODABCO2 with and Cl− CHanions2Cl2 and(producing DABCO-CH Cl ) 2inCl thecounter-ions presence of obtained the XB in − + donors [26]. Co-crystals 5 of CBr3NO2 with Cl− anions and DABCO-CH2Cl+ counter-ions obtained in donorsthis way [26]. showed Co-crystals 2D layers 5 of CBr (Figure3NO52 ).with Cl anions and DABCO-CH2Cl counter-ions obtained in thisthis way way showed showed 2D 2D layers layers (Figure (Figure 5). 5).

Crystals 2020, 10, x 7 of 15 CrystalsCrystals2020 2020, 10, 10, 1075, x 77 ofof 14 15

Figure 5. XB networks in (DABCO-CH2Cl)Cl ·CBr3NO2 co-crystals 5. Figure 5. XB networks in (DABCO-CH Cl)Cl CBr NO co-crystals 5. Figure 5. XB networks in (DABCO-CH2 2Cl)Cl· ·CBr3 3NO2 2 co-crystals 5. − The layers consisted of chair-like cells formed by 3:3 bonding of Cl and CBr3NO2 (similar to that The layers consisted of chair-like cells formed by 3:3 bonding of− Cl− and CBr3NO2 (similar − 3 2 obtainedThe withlayers CBR consisted3F or CBr of 4chair-like with Pr4NCl), cells formedwith Br by…Cl 3:3 bond bonding ...lengths of Cl of and3.1420 CBr (17)NO Å,(similar 3.1728 (16)to that Å to that obtained with CBR3F or CBr4 with Pr4NCl),… with− Br Cl− bond lengths of 3.1420 (17) Å, obtained with CBR3F or CBr4 with Pr4NCl), with Br Cl bond lengths of 3.1420 (17) Å, 3.1728 (16) Å+ 3.1728and 3.1715 (16) Å(16) and Å. 3.1715Stacking (16) of Å.the Stacking XB layers of created the XB channels layers created that are channels filled with that DABCO-CH are ﬁlled with2Cl and 3.1715 (16) Å. Stacking of the XB layers created channels that are filled with DABCO-CH2Cl+ DABCO-CHcounter-ions Cl(which+ counter-ions form also (which hydrogen form bonds also hydrogen with the chloride bonds with anions, the chloride Figure S1 anions, in the Figure Supporting S1 in counter-ions2 (which form also hydrogen bonds with the chloride anions, Figure S1 in the Supporting theInformation). Supporting Information). Information).The analogous reaction between DABCO and CH2Cl2 in the presence of tribromoacetamide led The analogous reaction between DABCO and CH2Cl2 in the presence of tribromoacetamide led to 2 2 to formationThe analogous of (DABCO-CH reaction 2betweenCl)Cl·CBr DABCO3CONH and2 crystals CH Cl 6. inThey the showedpresence multiple of tribromoacetamide types of halogen led formation of (DABCO-CH2Cl)Cl CBr3CONH2 crystals 6. They showed multiple types of halogen and andto formation hydrogen of bonding (DABCO-CH (Figure2Cl)Cl·CBr· 6). Most3CONH relevant2 crystals for the 6. current They showed work was multiple 2:2 bonding types of between halogen hydrogen bonding (Figure6). Most relevant for− the current work was 2:2 bonding between bromine bromineand hydrogen substituents bonding of CBr(Figure3CONH 6). Most2 and relevantCl anions for inthe which current each work chloride was 2:2is bondedbonding with between two substituents of CBr3CONH2 and Cl− anions in− which each chloride is bonded with two electrophiles electrophilesbromine substituents and two substituents of CBr3CONH of the2 and latter Cl that anions are bonded in which to twoeach Cl chloride−. The lengths is bonded of these with bonds two and two substituents of the latter that are bonded to two Cl−. The lengths− of these bonds were 3.0457 wereelectrophiles 3.0457 (12) and Å two and substituents 3.2770 (12) Å. of Thethe latterCBr3CONH that are2 moleculesbonded to aretwo also Cl . hydrogen-bondedThe lengths of these (via bonds the (12) Å and 3.2770 (12) Å. The CBr3CONH2 molecules are also hydrogen-bonded (via the NH2 group) − 3 2 − NHwere2 group) 3.0457 with(12) Åanother and 3.2770 Cl anion (12) Å.(Figure The CBr 6). Besides,CONH co-crystalsmolecules areshowed also hydrogen-bondeda halogen... Cl…Cl bond (via theof with another Cl− anion (Figure− 6). Besides, co-crystals showed a halogen Cl Cl− bond of− 3.2479 2 − … 3.2479NH group) (16) Å with between another chlori Cl anionne substituent (Figure 6). of Besides, DABCO-CH +co-crystals2Cl+ andshowed Cl aanions, halogen as Cl wellCl asbond two of (16) Å between chlorine substituent of DABCO-CH2Cl and Cl− anions, as− well as two hydrogen 3.2479 (16) Å between chlorine substituent of DABCO-CH2Cl+ and Cl anions, as well as two bondshydrogen between bonds the between cations the and cations chlorides. and chlorides. hydrogen bonds between the cations and chlorides.

+ Figure 6. Intermolecular interactions by chloride anions with tribromoacetamide and DABCO-CH2Cl Figure 6. Intermolecular interactions by chloride anions with tribromoacetamide and DABCO-CH2Cl+ in (DABCO-CH2Cl)Cl CBr3CONH2 co-crystals 6. (Note: blue line show contacts shorter than the sum + inFigure (DABCO-CH 6. Intermolecular2Cl)Cl· ·CBr interactions3CONH2 co-crystals by chloride 6. anions(Note: bluewith linetribromoacetamide show contacts shorter and DABCO-CH than the sum2Cl of van der Waals separations, counter-ions are omitted for clarity.). ofin van (DABCO-CH der Waals2 Cl)Clseparations, ·CBr3CONH counte2 co-crystalsr-ions are omitted 6. (Note: for blue clarity.). line show contacts shorter than the sum of van ...der Waals separations, counter-ions are omitted for clarity.). The Br Cl− − bond length measured in the solid-state XB associations prepared in the current The Br…Cl bond length measured in the solid-state XB associations prepared in the current work work together with− several representative examples of such bonds available in the crystallographic togetherThe withBr…Cl several bond lengthrepresentative measured examples in the solid-state of such XB bonds associations available prepared in the in crystallographicthe current work together with several representative examples of such bonds available in the crystallographic

Crystals 2020, 10, 1075 8 of 14 literature [44] are summarized in Table1. The Br ... Cl distances measured in the XB associations described above are within the range from 3.00 Å to 3.25Å. As such, they are 10% to 20% shorter than the van der Waals separations. The crystallographic database contains examples of XB associations showing substantially longer Br ... Cl bonds. For example, the complexes of chloride anions with bromopyridinium or dibromopyrrolidine cations show Br ... Cl distances in the 3.30 Å–3.50 Å range (which is close to the sum of the van der Waals radii of bromine and chloride of 3.66 Å [43]). On ... the other hand, the Br Cl distances in complexes of Cl− anions with N-bromosuccinimide or N-bromophthalimidesome of 2.70 Å–2.80 Å are closer to the covalent Br–Cl bond (2.14 Å [45]) than to ... the Br Cl− van der Waals separation.

Table 1. Halogen Br ... Cl bond length in the solid-state associations.

1 ... 2 R-Br Counter-Ion Bonding dBr Cl,Å Refcode + CBr3F Pr4N 3:3 3.0820 (10), 3.1266 (9), 3.1728 (10) This work + CBr3COCBr3 Pr4N 3:3 3.0306 (8), 3.0618 (8), 3.2056 (8) This work + CBr3CN Pr4N 6:3 3.0980 (4), 3.1320 (4), 3.1398 (4) This work + CBr4 Pr4N 3:3 3.0650 (7), 3.1769 (7), 3.2431 (7) This work + Et4N 4:4 3.090 VAPVOY + CBr3NO2 DABCO-CH2Cl 3:3 3.1420 (17), 3.1728 (16), 3.1715 (16) This work 2.987, 3.044, 3.269, Pr N+ 3:3 WOTTAC 4 3.124, 3.084, 2.986 + CBr3CONH2 DABCO-CH2Cl 2:2 3.0457 (12), 3.2770 (12) This work 3 + 3 C6F4Br2 C19H20NO 1:2 3.169 PUDBUO 2-BrPYR+ 4 - 1:1 3.310 CIGZIC 3-BrPYR+ 4 1:1 3.359 CIHBAX 4-BrPYR+ 4 1:1 3.312 CIHBOL + 5 C4H8Br2N 1:1 3.347 BRPYRL + 6 C12H8BrF2 2:2 3.049 YATMEO 7 + BrPIM Ph4P 2:1 2.700, 2.734 WEYZAB 8 + BrSIM Ph4P 2:1 2.822, 2.852 WEYYOO 2.603, 2.757, 2.801, ClBr Ph As+ 3:1 DIDHUW 4 2.632, 2.724, 2.730 + ClBr Et4N 1:1 2.362, 2.396, 2.410, 2.402 TEACBR + 9 ClBr Ph4P 1:1 2.426 GOLKAV + 10 Br2 NMHDA 1:1 2.567 URUBUF Br+ 2.139 11 - 1 Number of halogen bonds per chloride and XB donor. 2 CCDC refcodes, where available [38]. 3 1,4-dibromo-2,3,5,6-tetraﬂuorobenzene. 4 2-, 3- or 4-bromopyridinium. 5 Cis-3,4-dibromopyrrolidine. 6 Bis(4-ﬂuorophenyl)bromonium. 7 N-bromophthalimide [44]. 8 N-bromosuccinimide. 9 bis(Triphenyl 10 11 phosphine)iminium. N,N,N,N0,N0,N0-Hexamethylhexane-1,6-diaminium. Bond length in BrCl was measured in the gas phase [45].

It should be noted, however, that the Br ... Cl distances vary substantially in the solid-state associations of Cl− anion with the same R-Br electrophile (Table1). For example, the co-crystals ... of CBr4 with chloride show Br Cl distances from 3.07 Å to 3.24 Å, and those of CBr3NO2 show distances from 2.99 Å to 3.27 Å. Apparently, such bonding is characterized by a shallow energy minimum, and its length can vary considerable in response to crystal forces. Furthermore, each XB donor in these solid-state networks is bonded with several Cl− anions and each anion is bonded to several electrophiles. Such multiple bondings can noticeably aﬀect the XB length in the solid state [46]. Thus, to verify variations in the XB length derived from the X-ray crystallographic data, we carried out computational analysis of 1:1 XB complexes.

... 3.2. Quantum-Mechanical Computations of Halogen-Bonded R–Br Cl− Complexes ... Optimization of R-Br Cl− pairs (carried out in vacuum and in dichloromethane as a medium; see the Experimental section) produced energy minima showing Br ... Cl halogen bonds. While some of the R-Br electrophiles could also form hydrogen-bonded complexes with Cl− anions, this work is Crystals 2020, 10, 1075 9 of 14 focused on the minima showing single halogen bonding between reactants (with the X–Br–Cl angles in the range between 170◦ and 180◦, which is common for halogen bonding). The calculated interaction ... energies, ∆E, and the Br Cl− separations, dBr Cl, in the XB complexes calculated in dichloromethane ··· are listed in Table2. The energies of the complexes and individual components, X–Br–Cl angles and X–Br bond length, as well as the corresponding characteristics of the complexes obtained from the calculations in vacuum and atomic coordinates of all complexes are presented in the Supporting Information. For comparison, Table2 also presents characteristics of the BrCl molecule, which could + be formally considered as a result of interaction between a Br electrophile and a Cl− nucleophile.

Table 2. Characteristics of the calculated R–Br Cl complexes. · − 1 1 3 com sep 4 5 R-Br dBr Cl,Å ∆E, kJ mol− R dX–Br /dX–Br q(Cl), ··· CH Br 3.503 15.1 0.96 1.00 0.989 3 − CH BrF 3.415 11.9 0.93 1.00 0.985 2 − CH Br 3.395 9.9 0.93 1.00 0.982 2 2 − C H Br N+ 3.284 13.1 0.90 1.01 0.974 4 8 2 − − CHBr 3.223 4.5 0.88 1.00 0.967 3 − C Br F 3.219 2.5 0.88 1.00 0.969 6 2 4 − BrCCH 3.215 1.4 0.88 1.01 0.972 − 4-BrPYR+ 3.189 17.1 0.87 1.00 0.967 − − 3-BrPYR+ 3.185 17.2 0.87 1.00 0.966 − − CBr CONH 3.174 0.1 0.87 1.00 0.959 3 2 − − C Br F 3.167 4.8 0.87 1.00 0.962 2 2 4 − − CBr F 3.134 0.1 0.86 1.00 0.954 3 − CBrCl 3.127 0.6 0.85 1.00 0.950 3 − − CBr 3.122 0.5 0.85 1.00 0.948 4 − − CBr COCBr 3.106 1.5 0.85 1.00 0.948 3 3 − − CBr CN 3.057 5.0 0.84 1.00 0.934 3 − − CBr NO 3.038 5.7 0.83 1.01 0.93 3 2 − − C H BrF + 3.026 42.7 0.83 1.00 0.937 12 8 2 − − BrSIM 2.923 8.0 0.80 1.02 0.902 − − BrPIM 2.905 9.5 0.79 1.02 0.897 − − CBr(NO ) 2.887 18.7 0.79 1.02 0.888 2 3 − − BrSAC 2 2.556 21.4 0.70 1.08 0.682 − − Br 2.480 29.7 0.68 1.11 0.557 2 − − BrCl 2.427 42.0 0.66 1.12 0.513 − − BrF 2.416 73.7 0.66 1.10 0.551 − − pyrazine-Br+ 2.289 116.3 0.63 1.22 0.345 − − F Pyr-Br+ 2.179 209.8 0.60 1.49 0.147 5 − − Br+ 2.160 607.9 0.59 N/A 0.103 − − 1 From M062X/6-311+G(d,p) calculations with CH2Cl2 as the medium; see Supplementary information for details 2 3 and Table1 for R-Br abbreviations. N-bromosaccharin. R = dBr Cl/(rBr + rCl), where rBr and rCl are van der Waals radii of bromine and chloride or 1.85 Å and 1.81 Å, respectively [37··· ]. 4 Ratio of X Br bond lengths (where X = C, N, Cl, F or Br) in the separate and halogen-bonded R-Br electrophile. 5 Charge on the·· halogen-bonded Cl atom, from NBO analysis [47].

Comparison with the experimental data in Table1 demonstrates that the Br ... Cl distances in the complexes calculated in dichloromethane are mostly within the range of the Br ... Cl separations observed in the solid-state associations. The correlation between calculated and average experimental bond length in these complexes is characterized by an R2 of 0.94 (see Figure S2 in the Supporting Information). The Br ... Cl distances in the calculated gas-phase complexes are, on average, approximately 0.25 Å shorter than that in dichloromethane. As such, they are also substantially shorter than that measured for the corresponding solid-state associations (Table1). Further, ∆E values for the XB complexes of Cl− anions with the same R-Br molecule are more negative for the pairs calculated in vacuum than for the corresponding pairs calculated in dichloromethane. These results are consistent with the data reported earlier showing a higher bonding strength of the XB complexes involving an Crystals 2020, 10, x 10 of 15

Crystals 2020, 10, 1075 10 of 14 calculated in vacuum than for the corresponding pairs calculated in dichloromethane. These results are consistent with the data reported earlier showing a higher bonding strength of the XB complexes anionicinvolving XB an acceptor anionic when XB acceptor calculated when in calculated the gas-phase in the [15 gas-phase,22]. Further, [15,22]. while Fu mostrther, ofwhile the complexesmost of the calculatedcomplexes in calculated dichloromethane in dichloromethane and all complexes and calculated all complexes in the gascalculated phase are in characterizedthe gas phase by theare negativecharacterized∆E values by the (i.e., negative they are Δ thermodynamicallyE values (i.e., they are stable thermodynamically (Table S1 in the Supporting stable (Table Information), S1 in the theSupporting complexes Information), with the weakest the R-Brcomplexes electrophiles with calculatedthe weakest in dichloromethane R-Br electrophiles are characterized calculated byin thedichloromethane positive ∆E values are characterized (i.e., they are by only the kinetically positive Δ stable).E values (i.e., they are only kinetically stable). MostMost notably,notably, thethe datadata inin TableTable2 2and and Table Table S1 S1 in in the the Supporting Supporting Information Information show show a a gradual gradual changechange inin thethe XBXB Br Br...…ClCl length length from from the the values values which which are are close close to the sum of the van der Waals radiiradii ofof thesethese atoms atoms (R~1) (R ~ to1) thoseto those which which are 40%are shorter40% shor thanter thesethan separations.these separations. The latter The are latter within are 10%within of the10% covalent of the covalent Br–Cl bond Br–Cl (2.14 bond Å). The (2.14 decrease Å). The in decrease the Br ... Clin distancesthe Br…Cl is distances accompanied is accompanied by a progressive by a increaseprogressive in the increase interaction in the strength interaction (Table strength2 and Table(Table S1 2 and in the Table Supporting S1 in the Information). Supporting Information). Besides, in theBesides, stronger in the complexes, stronger the complexes, shortening the of shortening the Br ... Cl of distance the Br… isCl accompanied distance is accompanied by an elongation by ofan com sep com sep theelongation X–Br bond of the length X–Br (see bond the lengthdX–Br (see/dX–Br the dX–Brratios/dX–Br in Table ratios2 and in inTable Table 2 and S1 in in the Table Supporting S1 in the Information).Supporting Information). Importantly, Importantly, the appropriate the choice appropriate of R-Br ch electrophilesoice of R-Br inelectrophiles the series of in the the complexes series of the in Tablescomplexes1 and in2 (and Tables Table 1 and S1 in2 (and the SupportingTable S1 in Information)the Supporting allowed Information) eliminating allowed any eliminating large changes any betweenlarge changes subsequent between entries. subsequent As such, entries. the energiesAs such, and the interatomicenergies and distances interatomic in thesedistances series in cover these theseries whole cover range the betweenwhole range weak between van der weak Waals van complexes der Waals and complexes the covalently-bonded and the covalently-bonded BrCl molecule leavingBrCl molecule no substantial leaving gaps no substantial between subsequent gaps between entries subsequent (Figure7 entries). (Figure 7).

Figure 7. Relation between interatomic Br ... Cl distances and amount of charge transfer in the Figure 7. Relation between interatomic Br…Cl distances and amount of charge transfer in the R–Br·Cl− R–Br Cl complexes calculated in vacuum ( ) or dichloromethane (). complexes· − calculated in vacuum (○) or dichloromethane (□). # As the distance between Br and Cl decreased, substantial parts of the negative charge which was As the distance between Br and Cl decreased, substantial parts of the negative charge which was originally residing on the chloride anions were transferred onto the R-Br electrophile (the relation originally residing on the chloride anions were transferred onto the R-Br electrophile (the relation between the intermolecular distances and amount of charge transfer ∆q can be described by the between the intermolecular distances and amount2 of charge transfer Δq can be described by the equation dBr ... Cl= 0.303ln(∆q) + 2.2121 with R of 0.98; see Figure S4 in the Supporting Information). equation dBr…Cl= −0.303ln(− Δq) + 2.2121 with R2 of 0.98; see Figure S4 in the Supporting Information). Accordingly, the highest occupied molecular orbitals (HOMO) of the weak complexes are almost Accordingly, the highest occupied molecular orbitals (HOMO) of the weak complexes are almost completely localized on the chloride anions, and their lowest unoccupied molecular orbital is on completely localized on the chloride anions, and their lowest unoccupied molecular orbital is on the the R-Br electrophiles. The HOMOs and LUMOs of the strong complexes are delocalized over both R-Br electrophiles. The HOMOs and LUMOs of the strong complexes are delocalized over both counterparts (Figure8). As such, while the HOMO LUMO electronic transition in the weak complexes counterparts (Figure 8). As such, while the HOMO→ →LUMO electronic transition in the weak represents essentially a charge transfer from the donor to acceptor, the analogous transition in the complexes represents essentially a charge transfer from the donor to acceptor, the analogous strong complexes does not involve a substantial charge redistribution between the reactants. Finally, transition in the strong complexes does not involve a substantial charge redistribution between the due to substantial elongation of the N–Br bond in the complexes between N-bromopyrazinium and reactants. Finally, due to substantial elongation of the N–Br bond in the complexes between N- chloride and similar complexes in the bottom of Table2, these associations can be described as an bromopyrazinium and chloride and similar complexes in the bottom of Table 2, these associations association between a group R and a BrCl molecule (Figure8, right). This underlines the fact that as

Crystals 2020, 10, x 11 of 15 Crystals 2020, 10, 1075 11 of 14 can be described as an association between a group R and a BrCl molecule (Figure 8, right). This … theunderlines Br ... Cl distancethe fact decreases, that as the the Br intermolecularCl distance bond decreases, is gradually the inte transformedrmolecular into bond a covalent is gradually bond andtransformed the intramolecular into a covalent R–Br bond bond is converted and the into intramolecular an intermolecular R–Br one. bond It is importantis converted to stress intothe an continuousintermolecular character one. ofIt theseis important changes, to which stress underscore the continuous the direct character link between of these the changes, limiting typeswhich (intra-underscore and intermolecular) the direct link ofbetween bonds. the limiting types (intra- and intermolecular) of bonds.

Figure 8. Variations in the Highest Occupied Molecular Orbital (HOMO) and Lowes Unoccupied MolecularFigure 8. OrbitalVariations (LUMO) in the shapes Highest in theOccupied associations Molecu betweenlar Orbital R-Br (HOMO) electrophiles and and Lowes Cl− anions.Unoccupied Molecular Orbital ( LUMO) shapes in the associations between R-Br electrophiles and Cl− anions. 4. Conclusions 4. ConclusionsExperimental and computational studies on a series of complexes formed by R-Br electrophiles and chloride anions showed a wide variation of the structural and thermodynamic features of these Experimental and computational studies on a series of complexes formed by R-Br electrophiles associations. At the one extreme, the intermolecular Br Cl distances were within 5% of the sum and chloride anions showed a wide variation of the structural··· and thermodynamic features of these of the van der Waals radii of bromine and chloride, and the interaction energies were close to zero. associations. At the one extreme, the intermolecular Br···Cl distances were within 5% of the sum of Halogen bonding within these complexes was accompanied by a very small charge transfer and such the van der Waals radii of bromine and chloride, and the interaction energies were close to zero. intermolecular interaction practically did not aﬀect intramolecular bond length in the R-Br electrophiles. Halogen bonding within these complexes was accompanied by a very small charge transfer and such At the other extreme, the series comprised complexes in which the Br Cl distances were within 10% intermolecular interaction practically did not affect intramolecular··· bond length in the R-Br of a covalent Br–Cl bond. Charge delocalization between R-Br and Cl in these associations was electrophiles. At the other extreme, the series comprised complexes in− which the Br···Cl distances accompanied by the elongation of the intramolecular X–Br bond. As such, these complexes− could were within 10% of a covalent Br–Cl bond. Charge delocalization between R-Br and Cl in these be considered as an association of a neutral or anionic residue R with a BrCl molecule. The series associations was accompanied by the elongation of the intramolecular X–Br bond. As such, these under study contained complexes with characteristics covering the whole range of values between the complexes could be considered as an association of a neutral or anionic residue R with a BrCl limiting cases, with no substantial gaps between successive entries. It showed that the characteristics molecule. The series under study contained complexes with characteristics covering the whole range of the Br Cl bond can change gradually from the values typical for intermolecular associates to that of values··· between the limiting cases, with no substantial gaps between successive entries. It showed of a covalent Br–Cl bond. Together with the similar series of complexes between R-Br and DABCO that the characteristics of the Br···Cl bond can change gradually from the values typical for inter- or between diiodine and heteroaromatic N-oxides [6,7], and other systems [48,49] this indicates a molecular associates to that of a covalent Br–Cl bond. Together with the similar series of complexes generality of the continuum between the limiting types of bonding. between R-Br and DABCO or between diiodine and heteroaromatic N-oxides [6,7], and other systems It should be noted that the involvement of the weakly-covalent interactions in the formation of [48,49] this indicates a generality of the continuum between the limiting types of bonding. XB complexes represents a major topic of the ongoing discussions about the nature and properties It should be noted that the involvement of the weakly-covalent interactions in the formation of of halogen bonding (and many other supramolecular interaction, e.g., hydrogen bonding [50,51] or XB complexes represents a major topic of the ongoing discussions about the nature and properties of multicenter π-bonding between ion-radicals [52,53]). Indeed, the electrostatic (σ-hole) model explains halogen bonding (and many other supramolecular interaction, e.g., hydrogen bonding [50,51] or linear geometry and (if polarization is taken into account) energy variations in a large number of XB multicenter π-bonding between ion-radicals [52,53]). Indeed, the electrostatic (σ-hole) model explains complexes [9,10]. Yet, many computational studies indicated that covalent (charge transfer) interactions linear geometry and (if polarization is taken into account) energy variations in a large number of XB represent an important part of the bonding in many such associates [11–13]. Experimental studies complexes [9,10]. Yet, many computational studies indicated that covalent (charge transfer) of the XB complexes also revealed many features which can be explained by involvement of charge interactions represent an important part of the bonding in many such associates [11–13]. transfer. For example, photoelectron spectra of XB complexes and charge-density analysis directly Experimental studies of the XB complexes also revealed many features which can be explained by demonstrated charge transfer from XB acceptor (nucleophile) to XB donor (electrophile) indicating involvement of charge transfer. For example, photoelectron spectra of XB complexes and charge- covalency of halogen bonding [16,19]. Furthermore, appearance of a strong absorption bands in the density analysis directly demonstrated charge transfer from XB acceptor (nucleophile) to XB donor UV–Vis spectra of complexes and structural characteristics of many such associates suggest that the (electrophile) indicating covalency of halogen bonding [16,19]. Furthermore, appearance of a strong molecular-orbital interactions between XB species play a vital role in their formation [15,18]. While absorption bands in the UV–Vis spectra of complexes and structural characteristics of many such such features are usually most pronounced in the strong XB complexes, the data presented herein associates suggest that the molecular-orbital interactions between XB species play a vital role in their verify continuity of the bonding from the weak supramolecular associations to a covalent bond. This formation [15,18]. While such features are usually most pronounced in the strong XB complexes, the

Crystals 2020, 10, 1075 12 of 14 continuum implies an intrinsic link between the limiting types of bonding and, therefore, it suggests an onset of covalency in intermolecular interactions. The progressive increase in its input leads to the transformation of a weak supramolecular interaction into a covalent bond.

Supplementary Materials: The following are available online at http://www.mdpi.com/2073-4352/10/12/1075/s1, Figure S1: X-ray structure of (DABCO-CH2Cl)Cl CBr3NO2. Figure S2: Correlation between calculated and ... · experimental Br Cl distances. Figure S3. Correlation between calculated ∆E and ∆EINT. Figure S4. Correlation between calculated values of charge transfer and intermolecular distances. Table S1: Characteristics of the R–Br Cl− complexes calculated in the gas phase. Table S2: Calculated energies of the R-Br− electrophiles and the R–Br·Cl complexes. Table S3. Atomic coordinates of the calculated R–Br Cl complexes. · − · − Author Contributions: Conceptualization and methodology, S.V.R.; crystal preparation, S.V.R.; X-ray crystallographic analysis, M.Z.; quantum mechanical computations. C.L; data analysis, S.V.R.; writing—original draft preparation, S.V.R.; writing—review and editing, C.L., M.Z. and S.V.R.; visualization, S.V.R.; supervision, S.V.R.; funding acquisition, S.V.R. and M.Z. All authors have read and agreed to the published version of the manuscript. Funding: This research was funded by Division of Chemistry of the National Science Foundation, grant numbers CHE-1607746 and CHE-2003603. X-ray structural measurements were supported by the National Science Foundation through the Major Research Instrumentation Program under Grant No. CHE 1625543 (funding for the single crystal X-ray diffractometer). Acknowledgments: We thank Charlotte Stern (Northwestern University) for X-ray structure determination of (DABCO-CH Cl)Cl CBr NO co-crystals. 2 · 3 2 Conflicts of Interest: The authors declare no conflict of interest.

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